1. Field of the Invention
The present invention relates to an electric connector mainly used in digital signal transmission at a high speed and optimal for impedance matching with high accuracy.
2. Description of the Related Art
There are transmission systems of an unbalanced (single-end) type and of a differential type, which have been conventionally used in electric signal transmission at a high speed. In the unbalanced type, each signal path employs one signal line, the respective paths share one common ground line, and a voltage of the signal line is transmitted as a signal with a ground regarded as a reference. On the other hand, in the differential type, each signal path employs two signal lines, and a difference in voltage between the two lines is transmitted as a signal. The differential type is not responsive to signals such as electromagnetic noise, which are applied equally to its two signal lines, because voltages of the two signal lines are equal in amplitude and different from each other in phase by 180 degrees. Therefore, in comparison to the unbalanced type, the differential type is more sustainable with respect to noise and is suitable for transmission at a high speed.
FIG. 12 illustrates one example of an electric connector which is used in such a transmission system of the differential type. A body 500 is provided with terminal receiving holes 510, and upper and lower contacts 600a and 600b are respectively inserted into and retained by the terminal receiving holes 510. The upper and lower contacts 600a and 600b are arranged in a zigzag shape in a widthwise direction of the body 500, and base ends thereof are led out of a rear face of the body 500 and are substantially orthogonally bent downwards so as to be attached to an external circuit board (not shown).
Each of the upper and lower contacts 600a and 600b has a main portion to be inserted into one of the terminal receiving holes 510 of the body 500, a linking portion provided continuously from a rear end of the main portion to be substantially orthogonally bent along the rear face of the body 500, and a lead portion provided continuously from a rear end of the linking portion (for example, see Japanese Unexamined Patent Publication No. 2005-293970).
However, in the above-described conventional example, there is caused impedance mismatching between the adjacent contacts due to an offset between the upper and lower contacts 600a and 600b. Moreover, it is difficult to realize impedance matching at a high level, which is one of the major reasons for deterioration in transmission characteristics.
With the upper and lower contacts 600a and 600b, in a case where, as indicated in FIG. 12, each width of the main portions is set to 0.7 mm, a pitch distance between the adjacent main portions is set to 1 mm, the offset between upper and lower levels is set to 0.5 mm, and each width of the linking portions and the lead portions is set to 0.2 mm, then a pitch distance between the adjacent lead portions is equal to 0.5 mm. However, the upper contacts 600a are spaced apart from adjacent contacts 600a at a distance of 1 mm in particular portions from points where the linking portions of the upper contacts 600a are led out of the rear face of the body 500 and are bent at a substantially right angle to extend downwards to points where the linking portions of the upper contacts 600a are aligned in parallel with the linking portions of the lower contacts 600b. 
As described above, the distance between the adjacent contacts led out of the rear face of the body 500 is not entirely set to 0.5 mm but is partially set to 1 mm. Increased distance between the adjacent contacts causes decrease in electrostatic capacitance and increase in impedance. Another factor to decrease in electrostatic capacitance is that the linking portions of the upper contacts 600a are longer than the linking portions of the lower contacts 600b by the distance between levels of the upper contact 600a and the lower contact 600b, and that the upper contact 600a thus have larger areas that are not covered with the dielectric body 500 but are exposed to air.
As a result, the upper contacts 600a have larger impedances than those of the lower contacts 600b. Depending on the arrangement etc. of the contacts, the upper contacts 600a may have impedances significantly exceeding a predetermined standard value. Moreover, it is not easy to match impedances with high accuracy within a differential pair or between differential pairs. Such a problem is not unique to electric connectors of differential type but is also applicable to electric connectors of unbalanced type.